Nonwoven fabric

Manufacturing processes

Nonwovens are typically manufactured by putting small fibers together in the form of a sheet or web (similar to paper on a paper machine), and then binding them either mechanically (as in the case of felt, by interlocking them with serrated needles such that the inter-fiber friction results in a stronger fabric), with an adhesive, or thermally (by applying binder (in the form of powder, paste, or polymer melt) and melting the binder onto the web by increasing temperature).

Staple nonwovens

Staple nonwovens are made in four steps. Fibers are first spun, cut to a few centimeters length, and put into bales. The staple fibers are then blended, "opened" in a multistep process, dispersed on a conveyor belt, and spread in a uniform web by a wetlaid, airlaid, or carding/crosslapping process. Wetlaid operations typically use 0.25 to 0.75 in (0.64 to 1.91 cm) long fibers, but sometimes longer if the fiber is stiff or thick. Airlaid processing generally uses 0.5 to 4.0 in (1.3 to 10.2 cm) fibers. Carding operations typically use ~1.5 in (3.8 cm) long fibers. Rayon used to be a common fiber in nonwovens, now greatly replaced by polyethylene terephthalate (PET) and polypropylene. Fiberglass is wetlaid into mats for use in roofing and shingles. Synthetic fiber blends are wetlaid along with cellulose for single-use fabrics. Staple nonwovens are bonded either thermally or by using resin. Bonding can be throughout the web by resin saturation or overall thermal bonding or in a distinct pattern via resin printing or thermal spot bonding. Conforming with staple fibers usually refers to a combination with melt blowing, often used in high-end textile insulations.

Melt-blown

Melt-blown nonwovens are produced by extruding melted polymer fibers through a spin net or die consisting of up to 40 holes per inch to form long thin fibers which are stretched and cooled by passing hot air over the fibers as they fall from the die. The resultant web is collected into rolls and subsequently converted to finished products. The extremely fine fibers (typically polypropylene) differ from other extrusions, particularly spun bond, in that they have low intrinsic strength but much smaller size offering key properties. Often melt blown is added to spun bond to form SM or SMS webs, which are strong and offer the intrinsic benefits of fine fibers such as fine filtration, low pressure drop as used in face masks or filters and physical benefits such as acoustic insulation as used in dishwashers. One of the largest users of SM and SMS materials is the disposable diaper and feminine care industry.

Spunbond nonwovens

Spunbond nonwovens are made in one continuous process. Fibers are spun and then directly dispersed into a web by deflectors or can be directed with air streams. This technique leads to faster belt speeds, and cheaper costs. Several variants of this concept are available, such as the REICOFIL machinery. PP spunbonds run faster and at lower temperatures than PET spunbonds, mostly due to the difference in melting points

Spunbond has been combined with melt-blown nonwovens, conforming them into a layered product called SMS (spun-melt-spun). Melt-blown nonwovens have extremely fine fiber diameters but are not strong fabrics. SMS fabrics, made completely from PP are water-repellent and fine enough to serve as disposable fabrics. Melt-blown is often used as filter media, being able to capture very fine particles. Spunbond is bonded by either resin or thermally. Regarding the bonding of Spunbond, Rieter has launched a new generation of nonwovens called Spunjet.

Spunlace nonwovens

Spunlace nonwovens are made by a process called hydroentanglement. This process uses high-pressure jets of water to entangle fibers together, creating a strong, durable fabric. The process begins with a web of fibers, which can be natural or synthetic, laid out on a conveyor belt.

The web is then passed through a series of jets that spray water at high pressure onto the fibers. The water jets cause the fibers to tangle and interlock, forming a strong bond between them. The web is then dried and finished to create the desired product.

Flashspun

Flashspun fabrics are created by spraying a dissolved resin into a chamber, where the solvent evaporates.

Air-laid paper

Air-laid paper is a textile-like material categorized as a nonwoven fabric made from wood pulp. Unlike the normal papermaking process, air-laid paper does not use water as the carrying medium for the fiber. Fibers are carried and formed to the structure of paper by air.

Other

Nonwovens can also start with films and fibrillate, serrate or vacuum-form them with patterned holes. Fiberglass nonwovens are of two basic types. Wet laid mat or "glass tissue" use wet-chopped, heavy denier fibers in the 6 to 20 micrometre diameter range. Flame attenuated mats or "batts" use discontinuous fine denier fibers in the 0.1 to 6 range. The latter is similar, though run at much higher temperatures, to melt-blown thermoplastic nonwovens. Wet laid mat is almost always wet resin bonded with a curtain coater, while batts are usually spray bonded with wet or dry resin. An unusual process produces polyethylene fibrils in a Freon-like fluid, forming them into a paper-like product and then calendering them to create Tyvek.

Bonding

Several methods can be used:

• thermal bonding
  • use of a heat sealer
  • using a large oven for curing
  • calendering through heated rollers (called spunbond when combined with spunlaid webs), calenders can be smooth faced for an overall bond or patterned for a softer bond.
• hydroentanglement: mechanical intertwining of fibers by water jets (also called spunlace)
• ultrasonic pattern bonding: used in high-loft insulation, quilts and bedding.
• needlepunching/needlefelting: mechanical intertwining of fibers by needles
• chemical bonding (wetlaid process): use of binders (such as latex emulsion or solution polymers) to chemically join the fibers. An expensive route that uses binder fibers or powders that soften and melt to hold other non-melting fibers together
  • one type of cotton staple nonwoven is treated with sodium hydroxide to shrink bond the mat, the caustic causes the cellulose-based fibers to curl and shrink around one another as the bonding technique
  • one unusual polyamide(Cerex) is self-bonded with gas-phase acid
• melt-blown: fiber is bonded as air attenuated fibers intertangle with themselves during simultaneous fiber and web formation.

Disposability

The industry has attempted to define "flushability". They encourage voluntary testing of flushability by producers. They also encourage clear marking of non-flushable products as "No Flush" (rather than fine print on the bottom of products) including creating a "No Flush" logo.

The wastewater industry is encouraging a standard definition (rather than one which varies with each producer) of flushability, including dispersibility, and third-party assessment or verification, such as by NSF International. They believe that products should be safe for both septic and sewer systems (flushable and dispersible, respectively). Orange County Sanitation District has created a campaign, "What 2 Flush", which recommends flushing only the "three P's—pee, poop and [toilet] paper".